1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method.
2. Related Background Art
Along with progressive integration density and microminiaturization of semiconductor devices, including DRAM and other memory devices, the area occupied by each element has been reduced from one generation to another. For such enhancement of the integration density and microminiaturization, trench capacitors have often been used in memory cells. For further microminiaturization of semiconductor devices using trench capacitors, the diameter of the trench of each trench capacitor must be reduced.
FIG. 6 is a cross-sectional view of a memory cell region of an existing semiconductor device 300. The semiconductor device 300 includes a monocrystal silicon substrate 10, plate electrode 20, capacitor insulating film 30, collar insulating film 40, first storage node electrode 50, second storage node electrode 52, SiN layer or SiO2 layer 60, buried strap 70, element-isolating/insulating film 80, gate insulating film 90, source/drain diffusion layers 91, 92, gate electrode 93 and insulating residue 99. The storage node electrodes 50, 52 and buried strap 70 are made of doped polysilicon.
The gate insulating film 90, source/drain diffusion layers 91, 92 and gate electrode 93 make up a MOS transistor Tr. The plate electrode 20, capacitor insulating film 30 and first storage node electrode 50 are formed inside a trench 35 to function as a trench capacitor. Therefore, an electric charge coming through the transistor Tr, SiN or SiO2 layer 60 and the buried strap 70 is stored in the storage node electrode 50. Thus the corresponding data is written. On the other hand, the electric charge is released from the storage node electrode 50 via the SiN or SiO2 layer 60 to the transistor Tr. Thus the data is erased. Writing and erasure of data can be executed in this manner.
However, in case the diameter of the trench 35 e is reduced to enhance the integration density of the semiconductor device 300, an insulating residue 99 is produced. The insulating residue 99 may insulate the first and second storage node electrodes 50, 52 from each other, and may make it impossible to store the charge in the first storage node electrode 50 or release the charge from the first storage node electrode 50. This means that the device fails to write or erase data.
With reference to FIGS. 7A through 7D, the cause of the production of the insulating residue 99 is explained below. FIGS. 7A through 7D are cross-sectional views of the semiconductor device 300 in the order of its manufacturing steps to show the cause of the production of the insulating residue 99. FIGS. 7A through 7D show only the trench capacitor region, omitting the transistor region.
A silicon substrate 10 is processed by a known method to form the trench 35 and the plate electrode 20. The capacitor insulating film 30 is formed by oxidizing the inner wall of the trench 35. Next as shown in FIG. 7A, polysilicon 49 is deposited inside the trench 35. In case the trench 35 has a small diameter, a seam 97 remains in the polysilicon 49 when the polysilicon 49 is deposited.
As shown in FIG. 7B, the polysilicon 49 is next etched back by RIE (reactive ion etching). As a result, the first storage node electrode 50 is formed. During this etching, the etching gas not only licks the top surface of the polysilicon 49 but also intrudes into the seam 97. As a result, a V-shaped recess 98 as shown in FIG. 7B appears on the top surface of the first storage node electrode 50.
As shown in FIG. 7C., the capacitor insulating film 30 is next partly removed by etching using the first storage node electrode 50 as a mask to expose the upper part of the trench 35. Thereafter, a silicon oxide film is deposited on the inner wall of the trench 35.
As shown in FIG. 7D, the silicon oxide film is next partly removed by RIE to obtain a collar oxide film 40 of a desired thickness. Thereafter, polysilicon is once deposited and thereafter etched back by RIE to form the second storage node electrode 52.
However, in the process of forming the collar oxide film 40, the silicon oxide film is not removed from inside the V-shaped recess and remains therein as an oxide residue 99. As explained above, the oxide residue prevents electrical connection between the first and second storage node electrodes 50, 52.
If the etching is carried out more heavily to completely remove the oxide residue 99 in the process of forming the collar oxide film 40, then the collar oxide film 40 will become too thin. That is, the heavier etching results in making a sacrifice of appropriate adjustment of the collar oxide film 40 in thickness.
The recess 98 appears when the trench 35 is reduced in diameter and results in having a large aspect ratio. Therefore, reducing the aspect ratio could be a way of removing the recess 98. However, the increase of the diameter of the trench 35 for reducing the aspect ratio goes against the requirements of higher integration and microminiaturization.
Another countermeasure may be to use a semiconductor material having a lower melting point than silicon, such as germanium or silicon germanium, as the material of the storage node electrode 50 and level the recess 98 by annealing the storage node electrode 50.
However, in case a void 96 exists inside the first storage node electrode 50 (see FIG. 7B), the first storage node electrode 50, which is molten by annealing, may tumble into the void 96 and may peel off from the capacitor insulating film 30. Then the capacitance of the trench capacitor will decrease, or part of the first storage node electrode 50 will separate. As a result, the first storage node electrode 50 will fail to function as the electrode.
It is, therefore, desirable to provide a semiconductor device and its manufacturing method that ensure reliable connection between the first and second conductors in the trench and meet the requirements of higher integration and microminiaturization.